Simulated perspective display of aircraft landing strip on cathode ray tube



June 20, 1961 v. w. BOLIE 2,988,821

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT LANDING STRIP ON CATHODE RAYTUBE Filed Aug. 30. 1957 9 Sheets-Sheet 1 IV(4;,w F IE I 1 I I E 4 SIN(w t +80") INVENTOR.

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BY; Q6 Q g June 20, 1961 v. w. BOLIE smmmn PERSPECTIVE DISPLAY OFAIRCRAFT Filed 1m 30. 1957 9 Sheets-Sheet 2 finnwmsmhwdmwnmw mum 1NVENTOR.

VICTOR W 50L 1:

Arron #5 Y:

June 20, 1961 v. w. BOLIE 2,988,821

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT LANDING STRIP 0N CATHODE RAYTUBE Filed Aug. 50, 1957 9 Sheets-Sheet 3 SINGLE AX INVENTOR.

VICTOR W BoL/E MM MM A T TOR/V5 Y5 J1me 1961 v. w. BOLIE 2,983,821

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT LANDING STRIP ON CATHODE RAYTUBE Filed Aug. 30. 1957 9 Sheets-Sheet 4 INVENTOR.

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June 20, 1961 v. w. BOLIE 2,988,821

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT LANDING STRIP ON CATHODE RAYTUBE Filed Aug. 30, 1957 9 Sheets-Sheet 5 l J 44/41. 060E W, I 0/ VIPER6 7 f c I ANALOGUE W2 0/ woe/2 SU/JIM/IYG IVE TWORK T 64 T PERSPECTIVECONVEfZE/i 60 INVENTOR.

VICTOR W. 50111.:

June 20, 1961 v. w.

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT BOLIE LANDING STRIP ON CATHODERAY TUBE 9 Sheets-$heet 7 Filed Aug. 50. 1957 m w QNKRDNW/ u-l uINVENTOR. W B041:

V/croR BYM M Z Z June 20, 1961 v. w. BOLIE 2,958,821

SIMULATED PERSPECTIVE DISPLAY OF AIRCRAFT LANDING STRIP ON CATHODE RAYTUBE Filed Aug. 50, 1957 9 Sheets-Sheet 8 VERTICAL [44 PM FIE lE-(A)VICTOR W 50415 BYWMW A TTORAIEXS United States Patent Ofiice 2,988,821Patented June 20, 1961 2,988,821 SIMULATED PERSPECTIVE DISPLAY OF AIR-CRAFT LANDING STRIP ON CATHODE RAY TUBE Victor W. Bolie, Cedar Rapids,Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, acorporation of Iowa Filed Aug. 30, 1957, Ser. No. 681,319 4 Claims. (Cl.35-10.4)

This invention relates to means for perspectively displaying in twodimensions a three-dimensional geometric figure simulated by electronicsignals. Perspective viewing of the displayed figure can be providedfrom any visual angle.

Initially, the invention simulates a geometric figurethree-dimensionally by having three electrical signals, eachrespectively representing a single dimension of movement of a fictitiousgenerating point. Amplitude variations of the signals simultaneously acton the generating point so that it can move in any direction at anyinstant to generate the required figure. Therefore, each signal controlspoint movement along each of the three axes X, Y, or Z in the Cartesiancoordinate system. Initially, the simulated object is oriented to thecoordinates to enable the component signals to be generatedelectronically in the simplest possible manner. Hence, in the initialgeneration, viewing angle and perspective need not be considered.

A particular use for the invention is in visualizing an airfield runwayon the face of a cathode ray tube within the cockpit of an aircraft. Theinvention simulates a landing strip to appear in the same perspectiveand with the same viewing angle as it would appear to the pilot if hewere directly viewing the airstrip through the front window of theaircraft. In such case, the invention only needs information as to thepoint position of the aircraft from the landing strip, which can beobtained from such devices as a TACAN transceiver localizer and glidepath receivers and radio altimeter. Accordingly, under blind flyingconditions, a pilot can land an aircraft with much the same procedure asis done during clear weather conditions, permitting a view of thelanding strip.

The invention includes a pattern generator which provides three outputsignals that respectively represent the three-dimensional movements of agenerating point in the X, Y and Z coordinates. The pattern generatorpreferably generates signals representing the object when positionedmost simply with respect to the coordinates. For example, a cylinder maybe positioned with its axis coinciding with one of the coordinates, andthen a circle appears in the plane of the remaining coordinates. Acircle can be simulated by two single dimensional signals that are sinewaves of the same amplitude and 90 out of phase. The third signal is alinear sawtooth function, wherein the period of a sawtooth cyclecorresponds to the axial dimension of the cylinder. The rate of circularmovement of the generating point is high compared to its axial movement.A lower rate of circular movement will cause the cylinder to look like ahelix.

The invention further includes an axis-converter means whichelectronically modifies the three respective signals to alter theposition of the simulated figure with respect to the X, Y and Z axes,which have their position related to the ultimate viewing angle of theobject. Three adjustable inputs are provided to the axis-converter meansto provide three different angles of rotation. Thus, the axis converterelectronically operates upon the three respective single-dimensionalsignals to, in effect, permit a representation of the generated objectfrom another required viewing angle.

A perspective-converter means in the invention receives the three outputsignals from the axis-converter means and converts them into two signalscapable of acting on a generating point to create in two dimensions apicture of the initial object in perspective and from the viewing anglechosen for the axis-converter means. This generating point provides thedisplay and can be the point of light provided by a cathode ray tube(CRT). Hence,; the perspective-converter means changes the three signalsfrom the axis-converter means into two signals which can actuate thebeam of a CRT in a manner which displays the simulated pattern inperspective form at a viewing angle chosen by setting the controls ofthe axisconverter means.

Further objects, features and advantages of this invention will becomeapparent to a person skilled in the art upon further study of thespecification and the accompanying drawings, in which:

FIGURE 1 illustrates a principle of perspective views;

FIGURE 2 illustrates three-dimensionally a helical figure;

FIGURE 3 shows the pattern in FIGURE 2 viewed axially;

FIGURES 4(A), (B), and (C) are wave-forms of three single-dimensionalsignals electronically representing the helix shown in FIGURES 2 and 3;

FIGURE 5 is a block diagram of a form of the invention;

FIGURE 6 is a block diagram of one type of figure pattern generator;

FIGURES 7(A), (B), and (C) illustrate axis rotation operations withinthe invention;

FIGURES 8(A), (B) and 0 illustrate the three component axis controllingcircuits of one type of axis converter means of the invention;

FIGURE 9 illustrates a form of perspective-converter means;

FIGURE 10 illustrates a landing strip with respect to an aircraft;

FIGURES 11(A), (B) and (C) respectively represent electronically signalssimulating an airstrip viewed from the center of gravity of an aircraft,without regard to the attitude of the aircraft or the perspective of thelanding strip;

FIGURE 12 illustrates an object pattern generator capable of providingsignals shown in FIGURES 11(B) and (C);

FIGURE 13 illustrates a component of the apparatus of FIGURE 12;

FIGURE 14 is a block diagram of a system providing asimulated-perspective-pictorial display of an approached airstrip;

FIGURES 15(A), (B) and (C) show component single-axis rotation means forinjecting the attitude of the aircraft into the display; and,

FIGURE 16 shows another type of perspective converter.

Now referring to the drawings for a more detailed discussion of theinvention, FIGURE 1 represents the appearance of a distance rectangleO'M'NP' as viewed by an eye 10 upon a plane represented by axes 0W and0W Eye 10 is located a short distance D from the viewing plane; andrectangle OMN'P is located a distance W from the viewing plane W W Thus,the rectangle appears on the viewing plane as a smaller rectangle OMNPwhich has its dimensions related to the original rectangle as sides ofsimilar triangles which can be stated algebriacally by the followingformulas:

Although the transformation indicated by Expressions 1 and 2 is notlinear orthogonal, it does exhibit some linear properties. For example,a straight line apparent in the original pattern from the direction ofeye will appear as a straight line in its projection on the W W plane.In other words, it is obvious that a perspective transformation of astraight line is still a straight line although the length of theperspective segment may be altered.

The invention in effect simulates an object in the W W plane taking intoconsideration the distances D and W without necessarily requiring theexistence of an actual object such as represented by rectangle OMNP'.

For example, let it be assumed that it is desired to illustrate thehelix shown in FIGURE 2 while maintaining flexibility in viewing angleand proportioning of the helix. The helix is first simulatedelectronically by three signals representing the required movement of agenerating point as a function of time. This is done in its simplestmanner by orienting the axis of the helix with the S axis. Consequently,the generating signals represent the geometric figure viewed from itssimplest angle. Viewed axially along axis S which is perpendicular tothe paper in FIGURE 3, the helix appears as a circle. Thus, thegenerating point must move in a circle in the S 8 plane and movelinearly in the S direction. It is well known that a circle can begenerated by a Lissajous figure consisting of two sine waves equal inamplitude and 90 out of phase as illustrated in FIGURES 4(A) and 4(B).The third dimension of depth is a linear function of time which isprovided by the saw-tooth signal illustrated in FIGURE 4(C). The sinewaves have a frequency much higher than the repetition rate of thesaw-tooth wave.

A pattern generator shown in FIGURE 6 generates the signals of FIGURES4(A), (B) and (C). It includes a sine-wave oscillator 21 which providesoutput signal S and a 90 phase-shaft circuit 22 which is connected tothe output of oscillator 21 to provide a signal S that is 90 out ofphase with signal S A sawtooth generator 23 provides signal S The formof the invention in FIGURE 5 includes a pattern generator 20, whichmaybe of the type shown in FIGURE 6. A plurality of scalers 26, 27 and28 are respectively connected in tandem with the outputs S S and S ofgenerator 20. Each scaler is an attenuator, such as a potentiometer,which is used to adjust the level of its respective signal and controlsthe proportions of a simulated figure.

An axis converter 40 receives the scaled signals S S and S and modifiesthem so that the simulated figure ultimately viewed in perspective onthe face of CRT 30 appears from any desired viewing angle. Axisconverter is comprised of three component rotation means 41, 42 and 43.Each of these rotation means controls a single degree of the threedegrees of rotation necessary to obtain complete rotational control of aviewed pattern. In effect, each angle-axis rotation means provides ananalogue solution of an algebraic matrix which converts a point positionfrom a first set of three-dimensional axes to another set having acommon axis of rotation. Such single-axis conversion is illustrated ineach of FIGURES 7(A), (B) and (C). Thus, FIGURE 7(A) shows geomerticallya set of axes S S and --S rotated about axis S by an angle 6 to a newposition which corresponds to a new set of axes U U and -U Note thataxis U and axis S are common to both coordinate systems. The result ofthe transformation is that any signal repre sented in the coordinates ofS S and S is, after the single-axis transformation, represented in thecoordinates of U U and U Second single-axis rotation means 42 takes thenew set of axes U U and U and rotates them by an angle 0 about axis U toprovide a new set of coordinates V V and --V;; as shown in FIGURE 7(B).

The third single-axis rotation means 43 provides a rotation about thethird axis V by an angle 0 to provide the final transformation to thecoordinates W W and W Thus, the coordinate system represented by axes WW and W can be shifted by three degrees of rotational freedom withrespect to axes S S and S Coordinate conversion is well knownmathematically, and can be accomplished by solving the following systemof equations. The conversion shown in FIGURE 7(A) is:

1= 1 U2=S2 COS 01+S3 Sin 01 U =-Sq sin 61+S3 COS 61 The followingconversion by rotation means 42 is given mathematically by the followingequations:

And the last single-axis conversion provided by means 43 is given by theexpressions:

One form of circuitry for obtaining rotation means 41, 42 and 43 isillustrated by FIGURES 8(A), (B) and (C) respectively. Sincedirect-current components are presumed involved, such as illustrated forthe signals of FIGURES 4(A), (B) and (C), the respective signals must behandled by direct-current amplifiers and devices. Single axis-rotationmeans 41 in FIGURE 8(A) receives signals S S and S at its inputterminals. Note from Equation 3 above that its output signal U isdirectly provided by its input signal S Thus, a through connection isprovided by lead 46a.

FIGURES 8(B) and 8(C) are constructed similarly to FIGURE 8(A) exceptthat the signal inputs and outputs are transposed as shown. Thus, athrough connection is provided between signals U and V in means 42;while a through connection is provided between signals V and W inrotation means 43, as is also illustrated by dotted lines within therespective rotation means in FIGURE 5.

A pair of resistor-type resolvers 47 and 55 are used to obtaintrigonometric operation on the direct-current type signals in each ofmeans 41, 42 and 43. Such resistor-type resolvers are well known in theart and are described in Introduction to Electronic Analogue Computersby C. A. A. Wass, pages 131-137. Induction type resolvers can also beused provided that the signals are modulated onto a carrier signal as isdone in another embodiment described below. Each resistance-typeresolver includes a pair of taps 48-49 or 5657. Each pair of taps ismechanically rotatable as a unit. Each tap picks off a voltage whichvaries as a trigonometric function of its rotational position. The tapsare fixed relative to each other at a mechanical angle of so that onetap provides a sine-function output while another tap provides acosine-function output of the shaft angle. Since negative signals arerequired by resistor-type resolvers to provide complete trigonometricfreedom, a pair of polarity inverters 51 and 52 respectively receive thesignals directed to the resolvers. The inverted polarity signals areconnected to opposite sides of the respective resolvers so thattrigonometric functions can be obtained for 360 of variation of angle 0.A pair of summing networks 53 and 54 add respective resolver outputs toprovide the analogue solutions of Equations 4 and 5 in FIGURE 8(A),Equations 6 and 8 in FIGURE 8(B), and Equations 9 and 10 in FIGURE 7(C).Thus, the summing networks 53(a) and 54(0) in FIGURE 8(A) providesignals U and U summing networks 53( b) and 54(1)) in FIGURE 8(B)provide ouputs V and V and summing networks 53(c) and 54(c) in FIGURE8(C) provide signals W and W 'Ihe axis-converter output signals W W andW are provided as inputs to a perspective converter 60* in FIG- URE 5which is shown in more detail in FIGURE 9. Perspective converter 60provides an analogue solution of Equations 1 and 2 above and convertsthe respective threedimensional signals W W and W into two-dimensionalsignals X and Y having perspective characteristics. In FIGURE 9 apotentiometer 61 receives signal W Its tap 62 is set to provide a signalequal to W /D. If the potentiometer resistance varies uniformly withlength, tap movement is related to D (distance of eye to viewing area)by a factor of l/D and may be calibrated accordingly Where D isadjustable. If one volt is taken as unity for an analogue computation ofEquations 1 and 2, a battery 63 is provided which provides one volt thatis added to the output of tap 62 by a summing network 64 to provide anoutput of Analogue dividers 66 and 67 respectively receive the inputs Wand W and divide each of them by the output of summing network 64 toprovide signals X and Y as outputs of perspective converter 60.

Signals X and Y are provided to the horizontal and vertical platesrespectively of CRT 30 to generate a perspective view of the simulatedpattern generated by pattern generator 20 and angularly positioned byaxis con verter 40.

Accordingly, where helix signals generated by the device in FIGURE 6 areused, a helix appears in perspective on the face of CRT 30 with anyviewing angle selected by rotating the knobs of axis converter 40.

Object pattern generator 20, which produces the signals for a helix, canalso be made to produce signals to represent a cylinder. This is done bymaking the frequency of its output signals 8; and S very high withrespect to the repetition rate of the sawtooth wave signal S In suchcase, the lines of the helix become so close together that they appearas a continuous surface.

Any type of geometric pattern or object can be made to appearperspectively on the face of CRT 30 by designing object patterngenerator 20 to generate three-dimensional signals with respect to thethree directions of a coordinate system, with the simplest arrangementof signals generally being preferable.

Perspective is useful in judging depth (distance perpendicular to theviewing surface) of a two-dimensional display. Thus, in the case oflanding an aircraft, the pilot views a landing strip as he approaches itand judges distance and angle of descent by the apparent size andperspective shape of the landing field, as well as other objects aboutthe landing field.

The invention teaches how the geometric configuration of a landing stripcan be simulated on the face of a CRT located in an airplane cockpit sothat the landing strip is presented in perspective and position in muchthe same manner as it would appear to a pilot looking through hiscockpit window while landing the aircraft. Such a CRT presentation canenable a blind landing during zero visibility conditions.

A situation is illustrated in FIGURE 10 of an aircraft 80 flying withrespect to an airstrip 81. The position of the aircraft with respect tothe landing strip can be illustrated by means of a set ofthree-dimensional coordinates S S and S wherein coordinate S passes downthe center line of the fi.ld, coordinate S is horizontal and prpendicular to coordinate S along the closest edge of the airstrip, andcoordinate S is vertical passing through the point 0 of intersection ofS and S The position of the aircraft in this coordinate system is givenby the position of its center of gravity CG, which has the positionh,-a,-d; where h is the altitude of the aircraft, -a is the distance ofthe aircraft from point 0 projected on coordinate S and d is thedistance of the aircraft projected on coordinate S The information ofthe location of CG with respect to point 0 is made available by means ofsources such as distance measuring equipment (TACAN) and a radioaltimeter. Many other ways are possible for determining various of thecoordinate positions of point CG. Thus, localizer radio information canbe used to compute distance a, or VOR bearing information in conjunctionwith DME determined distance d can be used to compute distance a.Furthermore, this information can also be obtained from a ground radarinstallation and be continuously transmitted to the aircraft by means ofdata communications.

Thus, it is apparent that an aircraft can have available to it thecoordinates h, a, and d of point CG in the cordinate system S S and SHowever, this information is insufficient from the viewpoint of thepilot, because it does not consider the attitude of the aircraft. Thisinformation may be analogized to that obtainable from an upright personpositioned at the center of gravity looking at the field. However, thepilot is seated in a relatively fixed position with respect to theaircraft and has the attitude of the aircraft when looking forwardthrough the cockpit window. The portion of the window through which heviews the landing strip provides him with the relative position of thestrip with respect to the aircraft. In other words, the pilot views thelanding strip with respect to a set of coordinates that are fixed withrespect to the aircraft. These coordinates are designated W W and W inwhich W passes along the fuselage axis of the aircraft, W passes throughCG perpendicular to coordinate W and symmetrically through the wings ofthe aircraft, and coordinate W passes through CG perpendicular to bothcoordinates W and W Hence, the position of the airstrip with respect. tothe aircraft is determined by the pilot (often subconsciously) utilizingboth of these coordinate systems. The three-dimensional angle betweenthe coordinate systems is a function of the pitch 0, bank and heading 1of the aircraft as illustrated in FIGURE 10. The pitch and bank angles 0and are obtained from a vertical gyro located on the aircraft, and theheading information is obtained from the aircraft compass, since thedirection. of the airstrip center line 8;, is known. The information 0,4), and 1 causes the landing strip to be viewed by the pilot through aparticular portion of the window. That is, the landing strip will appearto the left, to the right, above or below the center of the window,according to the attitude of the aircraft.

The invention utilizes the two types of information (h, a, d and 0, -1to stimulate a landing strip in perspective and with the proper relativeposition on the face of a CRT located in the cockpit to provide a viewof the landing strip in much the same manner that it would appear to thepilot looking through the front window of his aircraft. Thus, when theairfield is not visible, such as during fog or low ceiling, the pilotcan guide the aircraft to a landing by viewing the CRT.

FIGURE 14 illustrates a form of the invention providing this type ofpresentation on the face of a CRT 1 30.

In the prior embodiment of FIGURE 5, the viewer was always presumed inan upright position and only the coordinates S S and 8;; were necessaryabout which to choose a viewing angle. There, the desired viewing anglewas known or was determinable by adjustment until the object appearedwith the required viewing angle. This cannot be done in the case of theairplane landing strip, since the position of the strip is unknown tothe viewer until he sees it on the face of the CRT. However, therequired viewing angle is determined by the attitude coordinatesrelative to the aircraft position coordinates S S and S Since theviewing angle in the coordinates S S and S is immediately known, and isa relatively simple type of information, it can be directly provided toa pattern generator 120 so that the signals representing the airstripcontain this viewing-angle information and are signals S S and S inFIGURE 14. Perspective is not considered by the landing-strip patterngenerator 120. An example of pattern generator 120 is given in FIGURE12. Note that altitude information h is passed directly throughgenerator 120 and accordingly is not altered by it, although it ismodulated onto a carrier. The landing strip pattern signals aregenerated photoelectrically by a pair of photoelectric cells 121 and 122which are separated from an elongated light source 123 by a cylindricalopaque card 124, which has light openings 126 and 127. Cylinder 124 isrotated by a motor 128. The shapes of openings 126 and 127 of core 124are shown in FIGURE 13. It is well-known that the intensity of thesignal increases as the width of the slot increases. Accordingly, theoutput from slot 126 will vary with signal 91 in FIGURE 11(B); and thesignal generated from slot 127 will vary with signal 92 in FIG- URE11(C). The variation and timing of signals 91 and 92 will move afictitious generating point to inscribe a rectangle that simulates arunway or landing strip. Motor 128 rotates at sufiicient speed (above 16revolutions per second) so that the generating point outlines thepicture of the landing strip at a fast rate compared to movement of theaircraft and the persistence of the human eye. Since the position of theaircraft is not presumed to substantially change during the period of asingle inscription of a landing strip pattern, the position signals 11,a, d shown in FIGURES 11(A), (B) and (C) are steady D.C. signals. TheDC. value of signal 91 represents distance a in FIGURE 11(B). Similarly,the DC. value of signal 92 in FIGURE 11(C) represents distance d.Consequently, the effect of the position of the CG in coordinates S Sand S effects the direct-current components of the signals and can beeasily superimposed upon the alternating current components generated byrotation of cylinder 124. This is done in FIGURE 12 by the respectivesumming networks 131 and 132. i

In practice, it is generally easier to handle information modulated onan A.C. carrier than it is to directly handle direct-currentinformation. Accordingly, modulators 133, 134 and 135 are proveded inFIGURE 12 to respectively modulate signals S S and S onto a 400cycle-persecond carrier. Such modulators are well-known in the art andare provided, for example, by choppers. Hence, alternating-currentamplifiers can be utilized as Well as alternating-current trigonometriccomputers such as wellknown resolvers.

Again referring to FIGURE 14, an axis converter 140 (basically similarto axis converter previously described) receives the respective outputsS S and S from landing strip pattern generator 120. Converter 140 iscomprised of three single-axis rotation means 141, 142 and 143 of thesame type specified in FIGURE 5, except that the system of FIGURE 14handles alternating-current modulated information instead ofdirect-current information.

Each of the three rotation means in FIGURE 14 is operated by anaircraft-attitude sensing device. Accordingly, a vertical gyro 144provides a bank-angle input 146 to rotation means 141, and provides apitch-angle input 147 to second rotation means 142. Similarly, a compasscourse indicator provides a heading-error angle input 148 to rotationmeans 143.

Circuitry comprising rotation means 141, 142, and 143 is shownrespectively in FIGURES 15(A), 15(B) and 15(C) which respectively solvethe three groups of Equations 3 through 11 given above. Each rotationmeans ineludes a pair of resolvers 150 and 156 which have their rotorscoupled to the respective angle input shown in FIGURE 14. Each resolverincludes two rotor windings 8 displaced by so that one provides anoutput that is a function of the sine of its shaft angle, while theother winding provides a cosine signal output. A pair of summingcircuits 153 and 154 are also included within each rotation means,wherein summing means 153 has its inputs connected to the outputs ofrotor windings 152 and 158, and the signal inputs to summing circuit 154are connected to rotor windings 151 and 157. Thus, rotation means 141solves Equations 4 and 5. Similarly connected components are shown inFIGURE 15(B) for rotation means 142 to provide the required solutions toEquations 6 and 8, and FIGURE 15(C) illustrates similar connections forrotation means 143 to solve Equations 9 and 10.

The outputs W W and W from axis converter are provided to a perspectiveconverter 160, shown in FIGURE 16, which translates the nonperspectivethreedimensional signals into perspective two-dimensional signals.Perspective converter operates upon its input signals in the same manneras perspective converter 60 in FIGURE 9, except that the signalsprovided to converter 160 are modulated on an alternating-currentcarrier. Amplitude-modulation detector 161 receives signal W and detectsit to provide an output proportional to the modulation information ofsignal W Each of the signals W and W is connected to a respectivecomputer circuit 163 and 164 which comprises a triode R or R having itscathode connected to ground and having a resistor R or R connected inseries with its plate. The other end of resistor R of circuit 163receives signal W 7 Similarly, the opposite end of resistor R of circuit164 receives signal W The grids of both tubes R and R are connected tothe output of detector 161.

Computer circuit 163 provides an analogue solution to the followingequation:

analogue solution to The above Equations 12 and 13 are obtained byapplying Kirkoffs law to the respective circuits 163 and 164. The tuberesistance R and R is thus a function of signal W and the resistance ofresistors R and R is determined according to the distance of the pilotseyes from the viewing face of CRT 130. Equation 12 may be compared toEquation 1 above to see that circuit 163 solves Equation 1. Similarly,Equation 13 may be compared with Equation 2 above to show that circuit164 solves the respective requirements of Equation 2.

The outputs X and Y from circuits 163 and 164 are unsymmetrical withrespect to ground potential. Thus, means is provided to reduce the A.C.axis of the signal to ground level. This can be provided by transformermeans or capacitor means. The latter is shown in FIG- URE 16, wherein apair of blocking capacitors 167 and 168 couple an output resistor 170across resistor R in circuit 163, with one end of resistor 170 beinggrounded. Hence, the X signal across resistor 170 has its A.C. axisreduced to ground level.

In a similar manner, a resistor 173 having one end grounded is coupledby a pair of blocking capacitors 171 and 172 to opposite ends ofresistor R of circuit 164.

A phase detector 176 is connected across resistor 170 to detect thesignal X amplitude modulation on the carrier and provide an outputpolarity depending upon the polarity of the carrier.

Second phase detector 177 receives the output of resistor 173 andsimilarly amplitude detects signal Y while correlating phase withpolarity in the well-known manner. Each phase detector 176 and 177 maybe of the type described in Principles of Radar, M.I.T., published in1946, page 12-36, FIGURE 32. Accordingly, the horizontal and verticalplates respectively of a CRT 130 are connected to the two signals X andY to provide a perspective display of the approached airstrip, at aposition on the face of the scope dependent on the position of theairfield with respect to the attitude of the aircraft. Thus, if theairfield appears at the lower righthand side of the CRT, then thelanding strip actually is in that direction from the aircraft. Avertical line may be drawn bisecting the face of the scope to indicatewhen the aircraft is lined up with the field, which will occur when thescope line centers down the viewed runway. Furthermore, an artificialhorizon can also be provided on the face of the scope to provide theaircraft with pitch and roll information. The provision of an artificialhorizon is well-known in the art and is not described herein, and suchartificial horizon does not require any perspective display.

In the general case of the invention in FIGURE 5, there may be instanceswhere three-dimensional control of the viewing angle is not necessary.In such case, two dimensional control can be obtained by having only twosingle-axis rotation means instead of three within axis converter 40.Similarly, if only a single degree of rotational control is required ofthe viewing angle, only one single-axis rotation means need be used foraxis converter 40, thus simplifying its over-all structure. The scalingattenuators '26, 27 and 28 can be used to adjust the respectivedimensions of the perspective pattern viewed on scope 30. By having thescaling attenuators operate upon the respective signals before axisadjustment, distortion is prevented.

Although this invention has been described with respect to particularembodiments thereof, it is not to be so limited as changes andmodifications may be made therein which are within the full intendedscope of the invention as defined by the appended claims.

I claim:

1. Display means for perspectively displaying a pattern, comprising apattern generator providing three signals representing the movement of apoint in respectively different directions, withsaid pointcircumscribing said pattern, three sealers respectively receiving thesignals of said generator to adjust their proportions, an axis convertermeans receiving the outputs of said sealers, said axis converter meansrespectively rotating the coordinates of said signals to rotate saidpattern about at least a single axis, a perspective converter receivingthe output signals of said axis converter means, said perspectiveconverter including plural computers, each computer electronicallymultiplying one of said signals by a factor D and electronicallydividing their product by the sum of D and a common one of said signalsto provide a computed signal, and point-scanning means receiving thesignal of said com puters to move a point accordingly and present aperspective display of said pattern.

2. Perspective converter means for translating first,

second and third signals respectively representing pointmovement inthree dimensions to describe a required pattern, comprising first andsecond analogue dividers in cluding numerator inputs receiving saidfirst and second signals, a unit potential source, and resistor meansreceiving said third signal, a summing network connected to said unitsource and said resistor means to provide an output representing theirsum, denominator inputs of each of said analogue dividers beingconnected to the output of said summing network, the outputs of saidpair of analogue dividers representing point-movement required todescribe said pattern perspectively in two dimensions.

3. Landing-strip pattern generating means for exhibiting perspectivelyon. a cathode ray tube a simulated landing strip as it would appear froman aircraft, comprising means for generating first and second signalsrespectively representing longitudinal and transverse dimensionalpoint-motions that describe a rectangle simulating said landing strip,means providing signals proportional to the altitude, distance, and atransverse displacement of said aircraft with respect to said landingstrip, means for summing said generated transverse signal with saidtransverse-displacement signal, and means for summing said distancesignal with said generated longitudinal signal, signals S S and S beingprovided respectively by the outputs of said summing networks and saidaltitude signal, axis converter means for translating the axis of saidpattern represented by said three signals into output signals W W and Wsaid axis converter having three angular inputs 4;, 0, and 1', verticalgyro means providing a bank input g5 and a pitch input 0, and compassmeans providing a heading input -1 perspective converter means forreceiving said axis converter means output signals W W and W andproviding respective point-motion signals X and Y for moving a pointsource to describe said pattern in twodimensional perspectivepresentation, said perspective converter means including computer meansfor computing where D represents the distance between the viewingdistance to the perspective pattern.

4. A landing-strip pattern display means as defined in claim 3 includingmeans for modulating onto a carrier frequency the signals S S and S saidaxis converter comprising three single-axis rotation conversion meansconnected in tandem, each single-axis conversion means having first,second and third input terminals and first, second and third outputterminals, with one of said singleaxis conversion means having athrough-connection betweenits first input and output terminals, anotherconversion means having a through-connection between its second inputand output terminals, and the last conversion means having athrough-connection between its third input and output terminals; eachsingle-axis conversion means including an angular input, a. pair ofresolvers, with their inputs respectively connected. to the remaininginput terminals of the respective single-axis conversion means, eachresolver providing a pair of outputs, one being the sine of its angularinput and the other being its cosine, a pair of summing circuits, eachhaving a pair of inputs, with one summing circuit having one inputconnected to the sine output of one of said resolvers and its otherinput connected to the cosine output of the other of said resolvers, theinputs to said other summing circuit being connected to the remainingoutputs of said resolvers, and the outputs of said summing networksconnected respectively to the remaining output terminals of therespective axis conversion means.

References Cited in the file of this patent UNITED STATES PATENTS2,399,671 Gage May 7, 1946 2,425,950 Morrison Aug. 19, 1947 2,479,195Alvarez Aug. 16, 1949 2,576,818 Waynick Nov. 27, 1951 2,604,705Hissarich et al. July 29, 1953 2,648,782 Argabrite Aug. 11, 19532,780,011 Pierce et a1. Feb. 5, 1957 OTHER REFERENCES ElectronicInstruments, Radiation Laboratory Series, vol. 21, McGraw-Hill, 1948,pages 159-160 relied on.

Electronic Analog Computers, by Korn and Korn, 2nd ed., McGraw-Hill,1956, pages 330-34 relied on.

